1. Field of the Invention
The present invention relates to a video decoder, and more particularly, to a video decoder capable of effectively reducing noise included in a chrominance signal when separating the chrominance signal from a composite video baseband signal (CVBS).
2. Description of the Related Art
A SECAM video decoder receives a composite video baseband signal and divides the composite video baseband signal into a luminance signal (Y signal) and a color signal (C signal). The color signal is demodulated into chrominance signals Cb and Cr. Original color signals R, G and B are generated using the Y signal and the chrominance signals to form an image on a predetermined display device.
FIG. 1 is a block diagram of a conventional video decoder 10. Referring to FIG. 1, the video decoder 10 includes multipliers 11 and 21, low pass filters 12 and 22, cloche filters 13 and 23, sign check units 14 and 24, differentiators 15 and 25, sign decision units 16 and 26, sign check units 17 and 27, and multipliers 18 and 28. The video decoder may further include a summer 31, a square root unit 41, and an operation unit 51.
The video decoder 10 receives a frequency-modulated composite video baseband signal, and multiplies the received composite video baseband signal by cos 2πft using the multiplier 11 and sin 2πft using the multiplier 21, respectively. The output signals of the multipliers 11 and 21 are respectively transmitted through the low pass filters 12 and 22 and the cloche filters 13 and 23. Through this process, a carrier frequency used when the composite video baseband signal was frequency-modulated is removed.
The signals output from the cloche filters 13 and 23 are respectively transferred to the sign check units 14 and 24 and the differentiators 15 and 25. The sign check units 14 and 24 check the signs of the signals input thereto and the differentiators 15 and 25 differentiate the signals input thereto. The sign check units 17 and 27 check the signs of the signals differentiated by the differentiators 15 and 25, and the sign decision units 16 and 26 decide signs using the signals output from the sign check units 14, 24, 17 and 27. The signals differentiated by the differentiators 15 and 25 are respectively squared by the multipliers 18 and 28.
The squared signals are summed by the summer 31, the square root of the summed signal is obtained using the square root unit 41, and then the square root is input to the operation unit 51. Furthermore, the values of the outputs from the sign decision units 16 and 26 are input to the operation unit 51. The operation unit 51 performs a predetermined operation on the input values and outputs the operating result as a chrominance signal Db or Dr.
FIG. 2 is a graph illustrating the characteristic of a SECAM composite video baseband signal. As illustrated in FIG. 2, the composite video baseband signal has a Y signal component distributed in a wide band and a C signal component distributed in a specific band. Two chrominance signals Cr and Cb are alternately transmitted through an even line and an odd line to prevent crosstalk between them. That is, the Y signal and the chrominance signal Cr are transmitted through one of the odd line and the even line and the Y signal and the chrominance signal Cb are transmitted through the next line.
A process of decoding the SECOM composite video baseband signal is divided into a luminance processing operation for the Y signal and a color processing operation for the C signal. Here, the C signal separated from the composite video baseband signal may include the Y signal component distributed in a wide band. In this case, color noise is generated in chrominance signals Cr and Cb demodulated from the C signal so that desired colors cannot be represented on a display. The conventional video decoder cannot effectively remove the Y signal component included in the C signal, and thus the color representation characteristic is degraded due to color noise.